(i) Field of the Invention
This invention relates to a shielded cable used to carry an electrical signal and to an apparatus which acquires a bioelectrical impedance value or biological composition data by using the shielded cable.
(ii) Description of the Related Art
An apparatus which acquires a bioelectrical impedance value by supplying a high frequency weak or small current between any two points of a living body through electrodes and measuring a potential difference in this current path through electrodes or an apparatus which acquires biological composition data based on the bioelectrical impedance value or the measured potential difference is well known. The apparatus may use a plurality of electrodes connected to the main unit of the apparatus via electric cables so as to supply a high frequency current between any two points of a living body and/or measure a potential difference in this current path.
As the electric cables which connect these electrodes to the main unit of the apparatus, a single core cable having a single conductive core wire covered with an insulator has heretofore been used. However, the single core cable is liable to cause measurement errors since electrical signals passing through the core wire also pass through another cable through an electrostatic capacitance between the cables or dissipate into the ground through a stray capacitance between the cable and the ground. The degrees of these errors change because the electrostatic capacitance between the cables or the stray capacitance between the cable and the ground change according to the positions of the cables, thereby causing significantly poor measurement reproducibility. Further, these errors become large when relatively long cables are used (when the distance between the main unit of the apparatus and a living body to be measured is large) and become larger along with an increase in the frequency of an electrical signal used for measurements. In particular, an electric cable for measuring a potential difference which carries the potential signal of a living body has a very high impedance and is vulnerable to noise from the outside and susceptible to the influence of the noise. The influence causes errors in the absolute value of a bioelectrical impedance and the phase thereof. The latter (error in the phase) is liable to become larger along with an increase in the frequency of an electrical signal used for measurements.
As a method for suppressing the measurement errors, a so-called “active shield” method using a shielded cable as the electric cables is known (refer to Non-Patent Publication 1, for example). According to this method, a shield is provided around the circumference of a film covering a core wire and is driven by an electrical signal which is the same as or slightly smaller than an electrical signal passing through the core wire. Thus, since the core wire is shielded from the outside by the shield, the electrical signal passing through the core wire is not influenced by the electrostatic capacitance between the cables and the stray capacitance between the cable and the ground, and since the shield is so driven as to retain the same potential as the core wire, an electrostatic capacitance between the core wire and the shield apparently does not exist. As a result, measurement errors as described above are suppressed.
Further, with respect to a stray capacitance between an electric cable and the ground, the present applicant proposes a bioelectrical impedance measurement apparatus which has a high input impedance buffer circuit in the vicinity of electrodes used for measurement of potential difference and uses a shielded cable connected to a ground potential as electric cables which connect the electrodes to the main unit of the apparatus, thereby making it possible to avoid the influence of a stray capacitance between the cable and the ground (refer to Patent Publication 1).
Non-Patent Publication 1
Settle et al., “Nutritional Assessment: Whole Body Impedance and Body Fluid Compartments”, NUTRITION AND CANCER, 1980, vol. 2, No. 1, p. 72 to 80
Patent Publication 1
Japanese Patent Laid-Open Publication No. 2001-61804
The foregoing active shield has a problem that a drive circuit therefor requires a buffer amplifier which operates stably over a wide frequency band so as to obtain the effect of suppressing the measurement errors by the active shield stably, thereby making the cost of the apparatus high.
In general, a buffer amplifier with a capacitive load connected thereto is liable to cause high frequency parasitic oscillation and is often unstable. That is, since an active shield using a buffer amplifier itself constitutes a positive feedback loop, oscillation by positive feedback occurs between the input side and output side of the buffer amplifier if the gain of the buffer amplifier is larger than 1. To prevent the parasitic oscillation, the gain of the buffer amplifier must be equal to or smaller than 1. When such a buffer amplifier with a gain of +1 is to be achieved over a wide frequency band, the cost of the buffer amplifier increases, thereby making the cost of the whole apparatus high.
In addition, when the shielded cable is to be used in an apparatus which acquires a bioelectrical impedance or biological composition data, the buffer amplifier must operate stably over a wide band even if the load of a subject (living body) is not pure resistance and changes according to its impedance status. This further increases the cost of the amplifier.
Further, the active shield has a possibility that the shield itself acts as an antenna and irradiates therethrough electromagnetic wave noise generated inside the main unit of an apparatus to which the shield is connected to the outside. As a result, in the presence of other electronic devices, it may influence these other electronic devices.
Meanwhile, in the case of a shielded cable as disclosed in the foregoing Patent Publication 1, i.e., a shielded cable connected to a ground potential, since an electrical signal passing through a core wire is driven by a low impedance by providing a high input impedance buffer circuit in the vicinity of electrodes as in the bioelectrical impedance measurement apparatus of the foregoing Patent Publication 1, attenuation thereof is little. However, when the high input impedance buffer circuit is not provided in the vicinity of electrodes, an electrical signal passing through the core wire is more liable to dissipate into the ground via the shield connected to the ground potential along with an increase in the frequency of the electrical signal, thereby causing measurement errors.